The present invention is directed to an improved fireplace grate for burning solid fuel cleanly and efficiently in a fireplace. It is an improvement over the inventions of my previous patents U.S. Pat. Nos. 4,515,147 and 5,850,830, and is designed to provide a positive flow of heated air into the room from the fireplace.
As all areas of the nation strive for cleaner air, the focus of correction is shifting from primary industrial and vehicular sources of pollution to secondary sources of pollution, such as wood burning fireplaces. In many residential areas, a complete ban on wood burning fireplaces is being proposed. In certain geographical areas where the topography is such that pollutants collect and are not readily blown away, such as Denver, Colo. and Beaver Creek, Colo., restrictions on wood burning fireplaces have now been in place for many years.
Pollution is caused because fireplaces do not burn all of the fuel vapor escaping from the burning wood and these fuel vapors are transported up the chimney into the atmosphere. For clean combustion, thorough mixing of the air and fuel vapor is required to achieve intimate contact between oxygen and fuel molecules. In addition, the mixture must be at a temperature high enough to cause the reaction to occur and sufficient time must be available to allow the reaction to go to completion. The reaction rate increases exponentially with increasing temperature so it is highly desirable to generate high temperatures to complete the reactions in the shortest time possible before the reactants have escaped from the hot combustion zone of the fire and have been cooled below their ignition temperature by dilution with excess air or heat transfer to the surroundings.
Existing fireplaces use natural draft to bring oxygen to the burning surfaces of the wood as well as to cause mixing of the fuel vapor and oxygen in the combustion zone. Natural draft forces result from the buoyancy of heated air or combustion gas, bringing new air to the combustion zone. Unfortunately, these buoyant forces are extremely weak (normally less than 0.01 inches of water gage (IWG)) and result in very low velocities and turbulence levels over the burning surface. These low velocities limit the rate at which combustion can occur on the burning surface of the wood and also limit the mixing of the volatile fuel vapor with the combustion air. Because of the limited combustion rate on the wood surface and limited combustion of the volatiles in the combustion zone, temperatures in the normal combustion zone in existing fireplaces are usually too low to allow complete combustion of the fuel vapors before they escape from the combustion zone and are cooled still further.
A further problem arises because of the coupling of the fire with the natural draft. Wood in a fireplace does not burn with uniform intensity over its entire surface but rather has zones where it burns with high intensity and zones where very little combustion occurs. The zones with high intensity create the most buoyant forces and hence, the most intense local draft. The hottest part of the fire tends to become even hotter. Unfortunately, the cooler part of the fire, which badly needs a stronger draft to burn hotter and cleaner, is unable to generate that draft so it tends to cool further and pollute more. Near the end of a fireplace fire, when the hottest zones of the fire have all burned out, little draft is left to burn the smoldering logs yet remaining and to prevent large quantities of unburned volatiles from escaping into the atmosphere.
Measurements taken in a conventional open fireplace indicate that the combustion zone in which adequate combustion temperatures may be achieved is localized to within a few inches of the burning wood and often is found only in the interstices between logs where the radiated heat from the burning surface is retained in a cavity. Fireplaces burn with large amounts of excess air (2000% excess air was measured in one test) that dilute and cool the combustion gas below combustion temperature shortly after it leaves the surface of the fuel. Temperatures measured twelve inches above the fire in a fireplace are approximately 200 F, well below the nominal 1000 F needed to initiate the combustion reactions. Hence, for fireplaces, the only opportunity to achieve clean combustion by burning the volatile vapors exists while the vapors are in the combustion zone, immediately adjacent to the surface of the wood.
The reflecting surfaces in most fireplaces are either brick, stone or metal. These have high thermal mass and absorb heat for a considerable time after the fire is underway, thereby actually cooling the fire. Only after they have been thoroughly heated do they radiate any significant heat energy back into the fire. Practically speaking, they do virtually nothing to enhance the efficient burning of the fire and are detrimental to it during the early stages thereof.
In U.S. Pat. No. 4,515,147 (fully incorporated herein by reference) granted to me and Samuel J. Van Grouw, entitled xe2x80x9cClean Burning Grate for Fireplaces and Wood Stovesxe2x80x9d we have taught a fireplace grate which provides a uniform supply of forced air to the entire lower surface of the burning logs and the maintenance of a combustion zone provided by a bed of hot coals supported adjacent to the lower surface of the burning logs. However, even this highly efficient combustion enhancing device could be improved by an increase in combustion efficiency which would result in further pollution reduction and heating of the room in which the fireplace is located.
In U.S. Pat. No. 5,850,830 (fully incorporated herein by reference) granted to me and entitled xe2x80x9cHeat Reflector for use with Fireplace Grate for High Temperature Combustionxe2x80x9d, there is disclosed a modification to the design of my U.S. Pat. No. 4,515,147 patent by means of the addition of a low thermal mass radiant energy heat reflector. This design provides a two-fold improvement over my ""147 design. Namely, radiant energy is directed by a reflector into the combustion zone, increasing combustion efficiency, thereby resulting in further pollution reduction, and radiant energy is directed by the reflector into the room.
When burning a wood fire in a fireplace, the desired result is to obtain a pleasant appearance and provide a cheery warmth to the room in which the fireplace is located, while minimizing the escape of pollutants into the atmosphere. To this end, the present invention is an improvement over the fireplace grates of my prior designs as it is designed to propel heated air into the room. It is intended that the present invention may be used with or without the low thermal mass radiant energy heat reflector of my ""830 patent.
In the present embodiment, as well as in my prior designs, the primary objective of the present invention is to reduce the pollution from wood burning fireplaces. This is achieved by blowing air directly onto the fuel, thereby incresing the combustion temperature. Both designs offer the benefits of easy starting, better aesthetics from the fire, near maintenance free operation of the fire, reduction in the consumption of wood by eliminating the need for a second layer of wood on the fire, burning all logs completely to the end of the fire and burning damp wood or difficult species of wood easily. Also, because of the high temperatures existing in the combustion zone, radiant heat transfer into the room is greatly increased, as compared with conventional fireplace grates. Additionally, the low thermal mass heat reflector will reradiate heat into the room.
It is the primary objective of the present invention to obtain all the advantages of my prior designs and additionally providing a mechanism for blowing heated air into the room.
These and other features and advantages of the present invention will become more apparent upon a perusal of the following description wherein similar characters of reference refer to similar parts in each of the several views.